The present invention relates to a pulverized coal burner which transfers pneumatically and burns pulverized coal and, more particularly, to a pulverized coal burner which raises the stability of flame in a low load operation and suppresses occurrence of nitrogen oxides.
Pulverized coal burning is required to suppress occurrence of nitrogen oxides (hereunder, referred to as NOx) to a small amount. Most of the NOx generated during burning of pulverized coal is NOx generated by oxidation of nitrogen contained in the coal. In order to reduce the amount of NOx generated in burning, various constructions of pulverized coal burners have been proposed.
As a pulverized coal burner which decreases an amount of NOx generated in burning, there is a pulverized coal burner in which a reducing zone and oxidizing zone are formed, that is, a so-called flame-inside-two-stage-burning burner. Nitrogen in the coal is released into gas phase as cyanide hydrogen (HCN) and ammonia (NH.sub.3) during thermal decomposition of pulverized coal at an initial time of combustion. Those nitrogen compounds are oxidized to become NOx while they have an effect of reducing NOx in a low oxygen-concentration region. The flame inside two stage burning realized effectively, inside flame, is a reaction which reduces NOx with NOx precursors such as NH.sub.3, HCN. In the burning, a reducing zone is expanded by fuel-excess burning with air shortage around the pulverized coal burner in the flame and an oxidizing zone is formed by high oxygen concentration burning at a downstream side of the flame.
In order to raise the effect of the flame inside two stage burning, it is necessary to stably form flame of pulverized coal supplied from a fuel nozzle. Therefore, it has been tried to change the supply condition of pulverized coal and carrier air. A pulverized coal burner inside which a member is arranged to adjust the particle concentration is disclosed in JP A 63-21406, JP A 3-41571, JP A 3-110308 or JP A 4-24404, for instance.
However, in the flame-inside-denitration-type pulverized coal burner applied to a pulverized coal burning boiler, it is important to promote a reducing reaction in a reducing flame zone in order to reduce the concentration of emitted NOx. In order to achieve it, it is important to achieve fuel-excess burning and raise the ability of thermal decomposition of pulverized coal in the reducing flame zone by not only improving the construction for supplying combustion air but improving the construction for supplying pulverized coal.
In order to raise the operability of the pulverized coal burning boiler, a load is changed in a short time. In view of this point, it is important to expand the lower limit of operation of the pulverized coal burner to a low load.
However, in the pulverized coal burner, it is impossible to reduce a flow rate of pulverized coal particles flowing in a pulverized coal transfer pipe to a certain speed or less. Therefore, it is limited to reducing a flow rate of air supplied in the pulverized coal transfer pipe. When the flow rate of pulverized coal particles is too small, there are such fears that the pulverized coal particles sink in the transfer pipe, the transfer pipe is choked, and flames in the furnace flow back into the transfer pipe.
For this reason, when the flow rate of pulverized coal and the flow rate of air are decreased in order to expand the lower limit of an operation of the pulverized coal burner to a low load, it is necessary to keep the flow rate of pulverized coal feed (or carrier) air at a certain level and decrease a feed rate of pulverized coal, after the load is lowered to some level of load.
When the feed rate of pulverized coal only is decreased, the concentration of pulverized coal contained in the pulverized coal feed air becomes lower. For this reason, pulverized coal flame is formed away from the outlet of the fuel nozzle. This means that the pulverized coal burns after the pulverized coal and combustion gas are mixed with each other. As a result, it is difficult to form a reducing zone of NOx inside the flame.